Gas Hydrates are ice-like, solid crystalline compounds formed of water molecules and gas molecules, in which each gas molecule is included inside a cage formed of hydrogen-bonded water molecules with a three-dimensional lattice structure. Gas molecules used for forming gas hydrates generally include low molecular weight gases like O2, H2, N2, CO2, CH4 and H2S and inert gases like Ar, Kr or Xe. However, some higher hydrocarbons such as ethane, propane and butane and freons also form gas hydrates. Although gas hydrates are similar in structure to ice, they are stable at temperatures higher than 0 degree Celsius (the freezing point of water) and at pressures above atmospheric pressure. When hydrates are formed out of saline or polluted water, they exclude all salts and other impurities and when the hydrates are dissociated, pure gas and desalinated water are produced. Gas hydrates are used, particularly, in a desalination or water treatment apparatus, as a technology for removing salts and impurities from water.
There are certain types of deep gas wells producing gas by hydraulic fracturing (also known as “fracking”) of deep shale deposits with mixtures of water, particulates and chemicals under high pressure that produce very highly saline water with salinities ranging from 50000 mg/l to 150000 mg/l of salt. Presently, there is no cost effective means to desalinate this highly saline water and consequently it is often disposed of in a manner that is harmful to the environment, particularly surface waters and the biota therein. In addition, the lack of a cost effective desalination method for highly saline waters has caused many governments to either ban hydraulic fracturing outright or limit its application. However, once a cost effective method of desalination of fracking wastewater is available, government regulations can be expected to reflect the performance levels that it can achieve.
A cost effective desalination method for highly saline waters will have universal application in all kinds of other wastewater treatment problems also.
Different methodologies using formation and dissociation of gas hydrates are used for desalination/treatment of water, some of which are discussed in the following documents. However, none of these technologies disclose a technically and commercially effective method for desalinating water of such high salinity up to its maximum possible eutectic composition of 230000 mg/l.
A recent United States Patent Publication US 2011/0064643 discloses an apparatus for continuously producing and pelletizing gas hydrates using a squeezing operation of a dual cylinder unit in a reactor. The apparatus comprises of a gas supply unit; a water supply unit; and a reactor into which gas and water are respectively supplied from the gas supply unit and the water supply unit. The slurry is squeezed by a squeezing stroke of the upper and lower pistons configured into upper and lower cylinders.
U.S. Pat. No. 6,158,239 discloses a method of desalination seawater by adding methane into seawater at a depth exceeding 100 meters to form methane hydrate which rises to where it is decomposed into methane and water, and recovering desalinated water. Methane is recycled to depth to form more buoyant hydrate.
U.S. Pat. No. 6,991,722 discloses an apparatus for desalinating input water, comprising: a desalination fractionation installation having a hydrate formation region disposed at a lower portion and a hydrate dissociation region disposed at an upper portion thereof; a mixing chamber; an input water conduit which is arranged to provide input water to the mixing chamber and to the hydrate formation region; and a gas supply conduit which is arranged to provide hydrate former to the mixing chamber and to the hydrate formation region; wherein hydrate former is dissolved into at least a portion of the input water in said mixing chamber prior to being input into said hydrate formation region.
Another technique disclosed in U.S. Pat. No. 7,569,737 describes a method for removing salts and/or other dissolved materials from water, using co-flow injection technology to form hydrate slurry, rapid depressurization the hydrate slurry to convert it into an ice-like clathrate hydrate mass and melting the an ice-like clathrate hydrate mass to first remove encapsulated salt and then recover purified water contained therein.
Another U.S. Pat. No. 7,794,603 B2 discloses a method of purification of contaminated water, wherein the water to be purified is passed through a first pipe into a first container to obtain hydrate formation, followed by mixing with a hydrate-forming compound which is supplied via a second pipe, wherein a portion of the mixture of hydrate and contaminated water is recycled to the first container via a third pipe as hydrate-forming seed, and the rest is passed to a separator where the mixture is separated into contaminated water and a hydrate. The hydrate is then passed to a second container via a fourth pipe, wherein, in the second container, the temperature is raised so that the hydrate dissociates into pure water and hydrate-forming compound, the hydrate-forming compound from said second container is passed back to the first container for hydrate formation via said second pipe and the pure water is taken out as a product. However, this method and apparatus requires number of vessels and costly transfer of hydrates between the vessels which makes this method cumbersome, less efficient and cost intensive.
Masayoshi Takahashi et al, describes a method for making gas hydrate comprising generating ultrafine bubbles in an aqueous solution and spontaneously generating hydrate nuclei by self-compression and collapsing of the ultrafine bubbles See Masayoshi Takahashi et al, “Effect of Shrinking Microbubble on Gas Hydrate Formation, The Journal of Physical Chemistry, 107, No. 10, 2003.
U.S. Pat. No. 2,904,511 discloses a method and apparatus for separating water from aqueous saline solutions and producing purified water from sea water through use of hydrate-forming reactions. The method comprises introducing a stream of aqueous saline solution into a hydrate-forming and hydrate-decomposing system, wherein the stream of aqueous saline solution is interacted with the hydrate-forming substance in a hydrate-forming zone. However, the apparatus of '511 involves multiple vessels and allied components including hydrate-forming vessel, hydrate-decomposing vessel, a hydrate conveyer, means for introducing water, and means for recycling hydrate-forming gas, making the overall system complex, expensive, and difficult to operate.
Therefore, none of the known techniques that are disclosed effectively desalinate saline water of wide range of salinities ranging from low levels of salinity to the maximum possible levels of salinity up to eutectic saline composition. There is currently no cost effective method for desalinating such highly saline water, and furthermore, no desalination technology based on gas hydrates has been disclosed that can effectively desalinate such highly saline water. The known desalination techniques require high capital and operating costs (including energy costs). Furthermore, carrying out different operations of the gas-hydrate based desalination techniques, including hydrate formation, expulsion of concentrated higher saline/polluted water, hydrate washing and hydrate dissociation, in different tanks/containers further results in an increase in cost and complexity of the structure and a decrease in efficiency. Further, known water purification technologies also do not provide an effective and easy control of pressure and temperature conditions during operation of hydrate-formation and disassociation, which is an integral part of desalination or water treatment processes.
There is, therefore, a need for an apparatus and method for significantly improving the efficiency of water treatment by desalinating saline water or treating polluted water from low levels of salinity to the maximum possible levels of salinity and at the same time using safe and cost-efficient techniques to implement the method.